Method and apparatus for automatically tuning mechanical frequency sources



Jan. 25, 1966 G. RASCH 1 METHOD AND APPARATUS FOR AUTOMATICALLY TUNING MECHANICAL FREQUENCY SOURCES Filed June 13, 1963 3 Sheets-Sheet 2 INVENTOR GERHARD RASCH Jan. 25, 1966 c; RASCH 3,230,614

METHOD AND APPARATIjS FOR AUTOMATICALLY TUNING MECHANICAL FREQUENCY SOURCES Filed June 13, 1963 5 Sheets-Sheet 5 a m o N "i E lm m g a: ,9 wwl 3 3 n. E X (0 2 E I n: 4 E E [l- T: 'i' O o ""I n g I. o 5 2 I 2 4 2 O II MN :5 NW I- INVENTOR GERHARD RASCH terial removing tool such as a grinding wheel.

United States Patent IMETHQD AND A P-PKRATUS FOR AUTQMATI- G .MEwA'NI-CAL FREQUENCY s .0

Gerhard Resell, Markneulrirchen, Saxony, Germany, as-

lsiguor, by mesne assignments, to VEB 'Musilrinstrumenteubaue-Musima, Markneuldrchen, Germany .Fileil June13, 1916.3,Ser. No.,287,6l7 11 ,Claims. (Cl. 29-'169.5)

This invention relates to a method and apparatus for automatically 'tuning mechanical frequency sources, es-

pecially vibrating reeds for use with musical instruments.

Mechanical Efrequency sources for musical instruments 'employingvibrating reeds consist generally of a sounding plate provided with rectangular openings above which the reeds are secured. Generally, the reeds must be tuned to a frequency tolerance of plus or minus one cent (one ent equals one hundredth half tone) which corresponds to a'tolerance of plus ,or minus 6 1O cycles per second.

.Inthe'pieparatioh of tone producers such as vibrating reeds which are milled or ground, such accuracy cannot "be achieved. Therefore, the reeds are subjected to a sub- ;sequent tuning process generally accomplished in three operations. In the first two operations, which may be "considered preliminary, the coarse errors are eliminated.

"The final tuning must occur within the instrument itself,

ground in the middle, the spring action and rigidity of the reed is reduced to the detriment of the reeds tonal quality.

Accordingly, the main object of this invention is to avoid the above mentioned disadvantages by providing a method and apparatus for automatically tuning frequency sources without requiring highly skilled manual assistance.

Another object is to provide apparatus for tuning a QVibrjatory reed Without reducing the rigidity thereof.

Stillanother object is to provide apparatus capable of increasing the production of tuned frequency sources.

Yet another object is .to provide apparatus for producing tuned, mechanical frequency sources having generally improved tonal quality.

According to the invention, theseobject-s are accomplished by resonating areed riveted to a sounding plate,

'generating .an electrical signal having a frequency proportional to the vibration of the reed, and comparing the signal with a reference frequency. Depending upon the frequency deviation of thissignal from .the reference frequency, anlerror signal is generated which activates, during predetermined intenval-s, a lifting arrangement adapted to move the free end of the reed against a rotating ma- These intervals maybe determined by a switch which alternately conducts and blocks the passage of the error signal to the lifting arrangement. Consequently, the grinding means may be inhibited for a -period during which the vibrating reeds may be vibrated at its new resonant frequency and regain sompared with :the reference frequency. The process continues until the absence of an error signal indicates that the resonant frequency of the reed is equal to the reference frequency, at which point the reed is properly tuned.

3,230,614 Patented Jan. 25, 1966 As a further feature of the invention, the measuring and grinding intervals may be made dependent upon the magnitude of the deviation of the vibrating reed from the reference frequency, i.e. for higher frequency deviations a long grinding period is used, and when the magnitude of the error signals become small, the grinding intervals are decreased. In this manner it is possible to more accurately control the tuning process. For exam ple, in the event an accuracy of two cents is required, the maximum grinding time, for one interval when the reed frequency approaches the desired frequency, is not longer than that required for a two cent change. If it is desired to provide a compl tely automatic tuning process, the error signal generated in accordance with the invention may be used to measure the frequency of a reed in order to initiate a subsequent sorting process by which irn properly tuned reeds may be rejected.

The manner in which these and other objects of the invention are accomplished will become-more apparent with reference to the following specifications and drawings, wherein:

FIG. 1 represents a block diagram of the invention;

PEG. 2 is a schematic diagram of the control circuit of the invention; and

FIG. 3 is a schematic diagram of a circuit which may be used for sorting the tuned reeds.

Referring now to FIG. 1, a conventional untuned reed it) is shown fixed to a conventional sounding plate 11 by means of a rivet 12. As schematically illustrated, light from source 14 is focused by means of lens 15 onto a photocell 1 6. The system is arranged so that vibration of the reed will periodically interrupt the path of the light to photocell 16. Under such conditions, the photocell will produce an output voltage which fluctuates at the frequency of the vibrating reed. The reed may be caused to vibrate by any conventional means (not illustrated) such as ablownozzle or the like.

The fluctuating signal from photocell i6 is then fed to a control circuit 18 which in turn develops an error signal to activate a lifting solenoid 19. Energization of lifting solenoid 19 is operative to force vertical rod 20 upwardly against reed 1i) pushing the reed against a material removing tool such as grinding wheel 21 which is rotated as shown. When reed 10 is pushed against wheel 21 the light beam from source 14 is no longer interrupted, and after a predetermined interval, control circuit 18 deactivates solenoid 19 permitting reed 10 to fall to its initial position. The reed is caused to vibrate once again, and the process continues until the reed is properly tuned, at which time control circuit 18 can no longer generate an error signal.

As shown in FIG. 1, control circuit 18 includes an amplifier 26 operative to develop an alternating voltage at its output which is coupled to a tuned amplifier '28. The output of amplifier 28 is fed to a discriminator 30 which develops a DC. error signal on its output X proportional to the frequency deviation between the resonant frequency of the reed and a fixed or reference frequency. The reference frequency is selected by properly tuning the resonant circuits of tuned amplifier 28 and discriminator 30. The error signal from discriminator 30 is fed to a switch 3.; which is activated by a clock pulse generator 34. As will be explained below, clock pulse generator 34 has two outputs; one of which permits the measurement of the reed frequency while the other enables the above described grinding operation to take place. Thus, when a signal is present on the GRIND output of generator 34-, switch 32. is enabled to pass the error signal from discriminator 30 to the lifting solenoid 19. When a signal is present on the MEASURE output of generator 34, switch 32 is inhibited to afford ample time to vibrate the reed and measure the resonant frequency thereof.

As a feature of the invention, a regulator 36 is coupled between discriminator 30 and clock pulse generator 34 to control the time of occurrence of the output pulse. Thus, as the output of discriminator 3i) approaches zero, indicating that the reed is almost tuned, regulator 36 alters the period of generator 34, reducing the grinding periods to avoid the possibility that too much material might be removed from reed 10.

In FIG. 2 a schematic diagram of control circuit 18 is illustrated. The fluctuating signal across photocell 16, indicative of the resonant frequency of the untuned reed 10, is coupled through capacitor C1 to the input of a twostage amplifier 40. The output of amplifier 40 is A.C. coupled through resistor R9 and capacitor C3 to a conventional double ended clipper or limiter, comprising diodes D1 and D2 and DC. batteries B1 and B2 so that its output has an amplitude independent of the amplitude of the applied input pulse.

Pulses from amplifier 26 are then coupled to tuned amplifier 23 which includes pentode 42, capacitor C5 and transformer 44. The values of the capacitor and transformer mutual inductance are selected so that the amplifier will be tuned to the desired resonant frequency. The amplified alternating voltage from tuned amplifier 28 is applied through capacitor C6 to a discriminator 30 which generates a direct error voltage proportional to the frequency deviation of the reed from the desired reference level. The operator of such discriminators is old and well known and need not be discussed in detail. It is sufficient to note that tuned circuits L1, C8 and L2, C9 are tuned precisely to the desired reference frequency. If the input voltage to the discriminator from amplifier 28 has the same frequency as the reference, equal but opposite voltages are formed across output resistors R12 and R13 and the resultant output voltage at point X is zero. If the frequency of the vibrating reed is lower than the reference frequency, the voltage across resistors R12 and R13 will be unequal and the resultant voltage at point'X will be negative. The greater the deviation from the reference frequency, the greater will be the magnitude of the voltage at point X. Similarly, if the reed should be vibrating at a frequency above the reference frequency, the voltage at point X will go positive.

The clock pulse generator 34 in this embodiment of the invention includes a multivibrator comprising triodes 49 and 50 and associated RC circuits R29, C12 and R23, C14. Multi-vibrators-are common electronic oscillators in which the tubes alternately conduct for a period of time dependent upon their associated time constants. In the present case, since it is desired that the GRIND period be short compared to the MEASURE period, the above time constants may be adjusted so that tube 5% will con- 52 and 54 coupling the error voltage at point X to the electronic switch 32. 7

Switch 32 includes a DC. amplifier 56 which is biased into conduction during the MEASURE interval by voltage divider R28, R27, R26 and the positive bias on its grid through closed contacts 53 and 54. The output of amplifier 56 is coupled through cathode follower 58 to a thyratron 60 which energizes the lifting solenoid 19 if a negative error voltage is coupled to the control grid of amplifier 56 through contacts 52 and 54 during the GRINDING interval.

The regulator 36 comprises a bridge including resistors R45 and Rfi7 inthe upper branches, triodes 64 and 66 in the lower branches, and relay 68 as the diagonal branch. The grid of triode 66 is coupled through resistor R49 to the error voltage at point X of discriminator 30. It will be recalled that the magnitude of the direct voltage at point X is dependent upon the frequency difference between the vibrating reed and the reference frequency. Accordingly, the bias on the triodes may be set so that when the negative voltage at point X rises to a predetermined level towards a value of zero, triode 66 starts to conduct and destroys the balance of the bridge. When the bridge is unbalanced, current flows through relay 68 in the diagonal branch, opening contacts 69 and 77 which effectively switches .off capacitor C13 which is across capacitor C12 of the clock pulse multivibrator. This capacitance, reduction, changes the time constant of the multivibrator so that the GRIND period will be reduced to a period of time which would not permit material to be removed from the reed beyond the tolerance requirements.

The operation of the circuit is as follows. As reed 10 is caused to vibrate, it periodically interrupts the beam of light to photocell 16 which produces a fluctuating voltage having a frequency proportional to the resonant frequency of the untuned reed. The fluctuating voltage from photocell 16 is A.C. coupled through the double stage amplifier 40 to limiter D1, D2 from which a voltage is obtained having a frequency identical to that of the vibrating reed and an amplitude independent of the input voltage. This voltage is then amplified by tuned amplifier 2% and coupled to discriminator 30 which produces a negative output voltage at point X having a magnitude indicative of the difference in frequency between the vibrating reed and the fixed reference frequency.

When multivibrator 49, 5t) switches states, connecting contacts 52 and 54, the negative error voltage is coupled to DC. amplifier 56 driving the tube to cut-off. With tube 56 cut-off, the voltage on the plate of tube 56 goes positive causing cathode follower 58 to conduct. The cathode of tube 58 is coupled through resistor R30 to the grid of thyratron 69, so that as tube 58 starts conducting and the voltage on its cathode goes positive, thyratron 60 starts to conduct. When thyratron 60 ignites, it energizes solenoid 19 which presses reed 10 against grinding Wheel 21 as explained above in connection with FIG. 1.

Since reed it) does not vibrate during the GRIND interval, the lightray is not interrupted at this time, and. the voltage at point X rises toward zero depending upon the time constant of C10, R12 and R13. The GRIND phase is ended when multivibrator 49, 50 changes states causing relay 51 to apply a positive voltage to the grid of amplifier 56 through contacts 53 and 54.s0 that this tube.

again conducts. When the plate voltage of amplifier 56 drops, tube 58 is biased to cut-off causing the grid of thy- I ratron 60 to go negative with respect to its cathode stopping conduction to solenoid 19. Once again, an error voltage at point X is formed so that the process may continue until the resonant frequency of the reed corresponds to the preset reference frequency. When this frequency has been obtained, no voltage is formed at point X, thyratron 60 cannot be fired, and the solenoid is not energized to push the reed against the grinding wheel- As previously described, regulator 36 is operative when the fre quency deviations are small to change the time constant of the multivibrator so that the grinding interval will not last long enough to exceed tolerance limits.

It should also be noted that in the event the resonant frequency of the feed is above the reference frequency,

the voltage at point X goes positive and thyratron 60 canode follower 58' and thyratron 60' which also operate in a manner similar to that described above.

The error voltage from point X is fed to a vibrator '70 which produces a pulsating direct voltage on its output line 72. This pulsating signal is converted to A.C. by capacitor C16 and thereafter clipped by diode D3 so that only a negative voltage is applied through contact 52' of relay 51 to the grid of DC. amplifier 56. Conduction of amplifier 56 causes thyratro n 60 to fire as described above to energize a solenoid 74- which activates a drop pivot (not shown) to insert the reed being tested into a reject container. Simultaneously, the voltage on the output of cathode follower 58' is coupled to a second thyratron 78 and an RC circuit comprising capacitor C18 and resistor R35. The time constant of this circuit may be adjusted so that if a given number of consecutive deviations occur, capacitor C18v will be charged to a voltage sufiicient to fire thyratron 78. Ignition of thyratron 78 causes relay 80 to open its contacts 82 and 84 which may be adapted in an obvious manner to open the voltage supply and put the entire circuit out of commission. If desired, relay contacts 82 and 84 may also be included in a circuit which will give a signal when the system is shut down. Since thyratron 78 is driven by direct current, the tubes will remain ignited even after the grid voltage is decreased, and the tube can only be extinguished by interrupting its anode voltage by means of switch 86.

At very low frequencies, it is diflicult to achieve high circuit efficiency for the resonant circuits L1, C8 and L2, C9 of discriminator 30. In such a case, a conventional frequency multiplier may be inserted between photocell 16 and capacitor C1 of the input to amplifier 40. In this manner, the circuit may be tuned to a given multiple of the desired frequency and operated at the higher frequency.

In addition, various means may be used to detect the frequency of vibration of the reed other than the photoelectric means disclosed. Many other modifications of the invention will also be obvious to one skilled in the art, and the invention should not be limited except as defined in the following claims.

I claim:

'1. Apparatus for tuning a mechanical frequency source, comprising means for vibrating said source, means for generating an electrical signal having a frequency proportional to the frequency of vibration of said source, means for generating an error signal proportional to the difference in frequency between said signal and a predetermined frequency, and means responsive to said error signal for changing the physical dimensions of said source'in order to vary the resonant frequency thereof.

2. Apparatus for tuning a mechanical frequency source according to claim 1 including timing means for enabling said dimension changing means only during predetermined time intervals.

3. Apparatus for tuning a mechanical frequency source according to claim 2, including regulating means for decreasing the length of said time intervals when the resonant frequency of the source approaches said predetermined frequency.

4. Apparatus for tuning a vibratory reed, comprising means for vibrating said reed, means for generating an electrical signal having a frequency proportional to the frequency of vibrations of said reed, timing means, means responsive to said timing means for generating an error signal during a predetermined time interval, said error signal being proportional to the difference in frequency between said signal and a predetermined frequency, grinding means, and means responsive to said error signals for bringing said grinding means into contact with said reed during predetermined time intervals in order to raise the resonant frequency of said reed.

5. Apparatus for tuning a vibratory reed according to claim 4 including regulator means for varying said last named time intervals when the resonant frequency of the reed approaches the predetermined frequency.

6. Apparatus for tuning a vibratory reed according to claim 5 wherein said generating means includes light generating means and photoelectric sensing means responsive to the interruption of said light beam by said vibrating reed.

7. Apparatus for tuning a vibratory reed according to claim 6 wherein said generating means includes a tuned amplifier and a discriminator for producing an error voltage proportional to the difference in frequency between the vibrating reed and said predetermined frequency.

8. Apparatus for tuning a vibratory reed according to claim 7 wherein said regulator means comprises amultivibrator and means for changing the time constant of said multivibrator depending upon the magnitude of said error voltage.

9. A process for tuning a mechanical frequency source, comprising causing said source to vibrate at its resonant frequency and thereafter generating an electrical signal proportional to the difference in frequency between said vibrating source and a predetermined frequency, changing the physical dimensions of said source during predetermined intervals under the control of said error signal, remeasuring the resonant frequency of said source and continuing the process until the resonant frequency of said source is equal to said predetermined frequency.

10. A process for tuning a mechanical frequency source according to claim 9 including the step of altering the time interval during which the physical dimensions of the source are changed when the resonant frequency of the source approaches said predetermined frequency.

11. Apparatus for testing and sorting a mechanical frequency source, comprising means for vibrating said source, means for generating an electrical signal having a frequency proportional to the frequency of vibration of said source, means responsive during fixed intervals for generating an error signal proportional to the difference in frequency between said signal and said predetermined frequency, and means responsive to the error signal during other fixed intervals for sorting said sources into good or reject containers depending upon the magnitude of said error signal.

References Cited by the Examiner UNITED STATES PATENTS 1,446,095 2/1923 Karaus 29-1695 WHITMORE A. WILTZ, Primary Examiner. THOMAS H. EAGER, Examiner. 

1. APPARATUS FOR TUNING A MECHANICAL FREQUENCY SOURCE, COMPRISING MEANS FOR VIBRATING SOURCE, MEANS FOR GENERATING AN ELECTRICAL SIGNAL HAVING A FREQUENCY PROPORTIONAL TO THE FREQUENCY OF VIBRATION OF SAID SOURCE, MEANS FOR GENERATING AN ERROR SIGNAL PROPORTIONAL TO THE DIFFERENCE IN FREQUENCY BETWEEN SAID SIGNAL AND A PREDETERMINED FREQUENCY, AND MEANS RESPONSIVE TO SAID ERROR SIGNAL FOR CHANGING THE PHYSICAL DIMENSIONS OF SAID SOURCE IN ORDER TO VARY THE RESONANT FREQUENCY THEREOF. 